var/stack/trap: tweak stack trap execution function
var/stack/trap: tweak stack trap execution function

lib/modernish/mod/var/stack/trap.mm, _Msh_doOneStackTrap():
- Make the QRK_LOCALUNS/QRK_LOCALUNS2 workaround conditional upon
  the shell having one of these quirks.
- On ksh93 (NONFORKSUBSH), (ab)use 'ulimit' to make the stack trap
  always run in a real forked subshell, which avoids many bugs.

modernish/modernish